1. Field of the Invention
The present invention relates to multilayer ceramic capacitors and particularly to a multilayer ceramic capacitor including ceramic dielectric layers mainly composed of barium titanate crystal grains having pores therein.
2. Description of the Related Art
In recent years, size-reduction and higher capacitance are increasingly required for multilayer ceramic capacitors for use in small, high-performance electronic devices such as cellular phones. Therefore, reducing the thickness of and increasing the number of ceramic dielectric layers and internal electrodes to be stacked has been pursued. Such a multilayer ceramic capacitor includes ceramic dielectric layers and internal electrode layers each 3 μm or less in thickness, and the number of layers stacked is 100 or more. In order to reduce the cost required to increase the number of layers to be stacked, the internal electrode layers of a small, high-capacitance multilayer ceramic capacitor are usually composed of a base metal, Ni rather than a known material, Ag—Pd.
In such a multilayer ceramic capacitor including many thin layers, a finer barium titanate powder is being used to achieve a further thickness reduction of ceramic dielectric layers. Thus, various processes for obtaining finer barium titanate powder have been devised. The dominant process for preparing a barium titanate powder has been a solid phase process in which a barium carbonate powder and a titanium oxide powder are used as the raw materials. However, recently, wet processes for preparing barium titanate powder such as a hydrothermal synthetic process and a coprecipitation process are put into practice to replace the solid phase process. Especially, the hydrothermal synthetic process is known as a process for fine, highly tetragonal barium titanate powders.
Barium titanate powders prepared by the hydrothermal synthetic process are mixed with various additives such as a glass component and an anti-reducing agent and the resulting mixture is fired in a reducing atmosphere. Numerous pores are formed in all parts of barium titanate crystal grains, such as near the grain surface and in the center region of the grain (see FIG. 8). A multilayer ceramic capacitor having ceramic dielectric layers sintered using such barium titanate powders has low reliability in a highly accelerated life test involving application of DC voltage at high temperature.